The present invention relates to a method for growing a semiconductor film on a substrate using gases and also relates to a method for fabricating a semiconductor device using the film growing method.
Recently, new semiconductor or semi-insulating materials have been researched and developed vigorously to realize a semiconductor device with special functions, e.g., specially enhanced radio frequency, emission or voltage-breakdown characteristics. For example, silicon carbide (SiC) is a semiconductor, which is harder, is less likely to be eroded with chemicals and has a wider band gap compared to silicon (Si), and is expected to be applicable to next-generation power electronic devices, radio frequency devices and devices operating at elevated temperatures. However, SiC has a polycrystalline structure, in which not only crystal grains with the same crystal structure and different crystallographic orientations but also crystal grains with various crystal structures coexist. Specifically, crystal grains of SiC include 3C SiC crystal grains with a cubic crystal structure and 6H SiC crystal grains with a hexagonal crystal structure.
Thus, a method for avoiding the formation of such a polycrystalline structure and thereby growing a single crystal SiC film with excellent crystallinity was proposed in Japanese Laid-Open Publication No. 62-36813, for example.
FIG. 16 schematically illustrates a structure of a known vertical SiC crystal grower. As shown in FIG. 16, the crystal grower includes chamber 100, susceptor 101, support shaft 114, quartz tube 115 and coil 103. The susceptor 101 is made of carbon and provides mechanical support for a substrate 102. The support shaft 114 is provided to support the susceptor 101 thereon. The coil 103 is wound around the quartz tube 115 for inductively heating the susceptor 101 with radio frequency current. The quartz tube 115 is so constructed as to allow cooling water to flow therethrough. The crystal grower is also provided with a gas supply system 107, in which cylinders of various gases to be supplied into the chamber 100 are disposed, and a gas exhaust system 111, in which a vacuum pump for exhausting these gases from the chamber 100 is disposed. The gas supply system 107 and the chamber 100 are connected together via source gas, diluting gas and additive gas supply pipes 104, 105 and 106 for supplying the source gases, a diluting gas such as hydrogen gas and an additive gas like a doping gas, respectively. The source gas supply pipe 104 is combined with the diluting gas supply pipe 105 at a midway point and then the combined pipe is connected to the chamber 100. At respective sites of the source gas and diluting gas supply pipes 104 and 105 before these pipes are combined, flow meters 108 and 109 are provided to regulate the flow rates of the gases supplied therethrough. Another flow meter 110 is provided for the additive gas supply pipe 106 to regulate the flow rate of the gas passing therethrough. The gas exhaust system 111 and the chamber 100 are connected together via an exhaust pipe 112, which is provided with a pressure regulating valve 113 for controlling the pressure inside the chamber 100.
Hereinafter, it will be exemplified how to grow a single crystal SiC film epitaxially on the substrate 102 of silicon or SiC by a CVD process.
First, suppose a single crystal SiC film is to be formed on a silicon substrate. In such a case, hydrocarbon (e.g., propane) and hydrogen gases are introduced from above and into the chamber 100 and the pressure inside the chamber 100 is regulated at atmospheric pressure or less. Radio frequency power is applied to the coil 103, thereby heating the substrate 102 up to about 1200.degree. C. at the surface thereof. As a result, the surface of the substrate 102 is carbonized to grow a very thin SiC film thereon. Thereafter, the flow rate of the hydrocarbon gas supplied is reduced and a gas containing silicon (e.g., silane gas) is introduced into the chamber 100, thereby growing a cubic SiC film on the surface of the substrate 102.
Next, suppose an SiC substrate is used as the substrate 102. In such a case, a substrate having its principal surface slightly tilted from a (0001) plane (C-plane) by several degrees in the [11-20] direction (such a surface will be called an "off-axis (0001) plane") is often used. This is because 6H SiC single crystals can be grown on the off-axis (0001) plane, whereas 3C SiC twin crystals are usually grown on the exactly (0001)-oriented plane. By using an SiC substrate with the off-axis (0001) principal surface, an SiC film can be grown on the substrate 102 even without performing any carbonization process thereon if the temperature inside the chamber is set to 1500.degree. C. or more. In adding a dopant to the SiC film, a doping gas is introduced through the additive gas supply pipe 106 into the upper part of the chamber 100. For example, when an n-type doped layer should be formed, nitrogen may be introduced. In such a case, the flow rate of the doping gas is controlled at a desired concentration using the flow meter 110. And when the SiC film is formed by this crystal growing process, the supply of respective gases through the pipes 104, 105 and 106 and the application of radio frequency power to the coil 103 are both stopped, thereby cooling down the substrate 102.
A horizontal crystal grower, in which the quartz tube 115 of the chamber 100 is placed to have its axis extend horizontally, is also used. The crystal grower includes the same main members as those of the vertical crystal grower shown in FIG. 16. But in the horizontal crystal grower, various gases are supplied through one of the side faces of the chamber.
The prior art crystal film forming method, however, suffer the following shortcomings.
Firstly, if a MESFET, for example, is formed using a semi-insulating substrate of SiC that has been formed by the above method, then the MESFET cannot always realize expected radio frequency characteristics or performance. Specifically, in a MESFET formed to have a GaAs film grown epitaxially on the substrate by the conventional technique, as the gate length thereof is shortened to cope with increase in operating frequency of the device, the transconductance thereof decreases. This is probably because an increased amount of current leaks from a heavily doped channel layer into a non-doped underlying layer due to a less sharp dopant concentration profile in a transition region between the non-doped and channel layers. Thus, according to a suggested technique, the decrease in transconductance is suppressed by reducing the leakage current using a steeply rising dopant concentration profile that has been created in the channel layer through implantation of a dopant of the opposite conductivity type into the underlying layer.
Such defects involved with that non-sharp dopant concentration profile are noticeable in an SiC crystal film, too. We also found similar defects in crystal films made of various materials other than SiC and GaAs.
Secondly, if an SiC crystal film is grown homoepitaxially on an SiC substrate with the off-axis (0001) principal surface, then large level differences are often formed. FIG. 6(a) illustrates the surface of an SiC crystal film formed by the conventional method. In FIG. 6(a), a surface state near a micro-pipe, which often appears on an SiC substrate, is shown to indicate the heights of the level differences. As shown in FIG. 6(a), a great number of level differences with a width of several hundreds nanometers and a height of several tens nanometers are formed on the surface of the SiC crystal film and the planarity of the SiC crystal film is not so good. We found that those level differences are observable particularly noticeably in a crystal film that has grown homo- or heteroepitaxially on a substrate having its principal surface slightly tilted from a densest plane like a C-plane.
That is to say, we noticed that the surface state of an epitaxially growing thin film is not controllable at an atomic level according to any of the conventional gas supply methods.